Method and apparatus for analyzing nitrogen oxides in gas

ABSTRACT

Nitrogen oxides in a gas, for example, flue gas from a boiler, are continuously analyzed by (a) adding ozone gas to the gas containing nitrogen oxides thereby converting the nitrogen oxides to dinitrogen pentaoxide, (b) thermally decomposing the dinitrogen pentaoxide in the gas to nitrogen dioxide at a temperature of 300* to 340*C, and (c) continuously measuring nitrogen dioxide content of gas by an optical analyzer, for example, dual wavelength, photoelectric type analyzer of ultraviolet and visible regions.

United States Patent 1191 Komatsu [11] 3,835,322 1451 Sept.-10, 1974METHOD AND APPARATUS FOR 2,974,227 3/1961 Fisher et al. 250/3733,005,097 10/1961 Huminel 250/343 ANALYZING NITROGEN OXIDES GAS3,166,676 1/1965 Robinson 250/373 [75] Inventor: Isa u o Takahagt, Japan3,593,023 7/1971 Dodson et al 250/343 Assignee: Hitachi, Ltd. y Japan3,696,247 10/1972 McIntosh et al 250/345 [22] Filed: Feb. 2, 1973 vPrimary Examiner-James W. Lawrence Assistant Examiner-B. C. Anderson[21] Appl' 329119 Attorney, Agent, or FirmCraig & Antonelli [30] ForeignApplication Priority Data [57] ABSTRACT Feb. 4, 1972 Japan 47-12215Nitrogen oxides in a gas, for example, flue gas from a boiler, arecontinuously analyzed by (a) adding ozone [52] U.S. Cl 250/343, 250/345,250/340, gas to the gas containing nitrogen oxides thereby con- 250/373verting the nitrogen oxides to dinitrogen pentaoxide, [51] Int. Cl. G0ln21/00 (b) thermally decomposing the dinitrogen pentaoxide [58] Field ofSearch 250/343, 352, 373, 340, in the gas to nitrogen dioxide at atemperature of 300 250/345 to 340C, and (c) continuously measuringnitrogen dioxide content of gas by an optical analyzer, for exam- [56]References Cited ple, dual wavelength, photoelectric type analyzer ofUNITED STATES PATENTS ultraviolet and visible regions. 2,648,775 8/1953Waters 250/343 29 Claims, 4 Drawing Figures @14 I2 1 S l JOINTIIl-EATING MEAELRE- NT T FURNACE CELL I5 6 P3 FLOW METER I 5 z 4 0 ONEGENERATOR DUST 3 REMOVAL FLO MET FILTER w ER I COOLING N2 TRAP 7 OXYGENCYLINDER PAIENIEBSEP 1 0 I974 SHEET 1 BF 2 Fl G. l

@14 l2 1 JOINTII S ,5 l-EATING Ma a? FURNACE CELL T 6 f3 FLOW METER 5OZONE GENERATOR DUST x3 REMOVAL FLOW METER FILTER 9 COOLING m2 TRAP a 7OXYGEN CYLINDER This invention relates to a method and apparatus forcontinuously analyzing nitrogen oxides in a gas.

It is now necessary to measure the S content of flue gas as a source forair pollution, and it will surely be necessary in near future to measurethe NO,, content of the flue gas. Accordingly, a method and apparatusfor continuously analyzing No, with a high reliability have been keenlydesired.

At the present time, a few methods and apparatus for analyzing NO, havebeen proposed, but these have a problem in long-term, continuous use forthe analysis of the flue gas from stationary gas source such as boiler,etc., because frequent maintenance is necessary owing to drifts of NO,,detector itself or deterioration of devices constituting the apparatus,and reliability is considerably lowered thereby.

Therefore, it would be possible to apply a dual wave length,photoelectric type analyzer of visible region having a high reliabilityfora prolonged time without any maintenance to the NO analysis, wherethe N0 analysis is a base. However, NO, in the flue gas consists largelyof nitrogen monoxide, which will be hereinafter referred to as NO, andthus it is necessary to find out a method for converting NO to N0stably, continuously and with high efficiency without any maintenancefor a prolonged period and any generation of secondary pollution.

Several methods have been reported for continuously analyzing NO invarious sample gases, but a particularly important point in the reportedmethods is a stable conversion of NO to N0 with a high efficiency and ata rate as high as possible.

The reported methods include (1) a method comprising adding air oroxygen to a sample gas and compressing the gas for a predetermined time,(2) a method comprising passing asample gas through an oxidativesolution containing potassium permanganate, etc., and (3) a methodcomprising contacting a sample gas with ozone.

In the case of the method (l), it takes a few minutes to convert NO toN0 even if the gas is compressed to 3 kg/cm with oxygen, and thus themethod (I) is not suitable for continuous analysis. In the case of themethod (2), it is rather difficult to obtain a good conversionefficiency, and the oxidizing agent must be exchanged from time to time.Furthermore, there is a fear of generation of secondary pollution, suchas water pollution, by the waste oxidizing agent. In the case of themethod (3), as disclosed in Japanese Pat. Publication No. 21354/70(Method and Apparatus for Continuously Analyzing Nitrogen Oxides), NO,can be very rapidly converted to dinitrogen pentaoxide showing a strongabsorption at 5.8 p. of infrared absorption spectrum, when NO, iscontacted with ozone for reaction, and thus an infra-redspectrophotometer can be utilized. Dinitrogen pentaoxide will behereinafter referred to as N 0 However, the infra-red spectrophotometeris readily influenced with moisture of the sample gas, and undergoesdrifts. Thus, it is necessary to check the zero point and sensitivity ofthe infra-red spectrophotometer, at least once a week, with zero gas andstandard gas of known concentration. That is, there is a problem inmaintenance in the case of the method (3).

An object of the present invention is to provide a method and apparatusfor measuring NO, content of a sample gas by oxidizing NO, in the samplegas to N 0 with ozone according to the well known method, converting N 0to N0 stably, continuously and with a high efficiency without anymaintenance fora prolonged period of time and any generation ofsecondary pollution, and measuring N0 content by a dual wavelength,photoelectric type analyzer of visible region.

The present invention is based on the following chemical equations.Equations for contact reaction of N0 with ozone are given below(Japanese Pat. Pablication No. 21354/):

2NO2 03 N205 02 It is seen from the equations l) and (2) that NO and n N0 can be completely converted to N 0 when ozone is addedthereto in molesat least 1.5 times as much as that of NO and at least 0.5 times as muchas that of N0 Furthermore, the time required for the conversion to N 0is not more than about one second.

cooling 2NO 2NO+ O Therefore, it is seen from the equations (3) and (4)that there is an optimum heating temperature in the decomposition of N 0to N0 Now, the present invention will be explained in detail by way ofthe accompanying drawings:

FIG. 1 shows one embodiment of the present invention.

FIG. 2 shows an absorption spectrum diagram of nitrogen dioxide.

FIG. 3 is a diagram showing a relation between the heating temperatureof dinitrogen pentaoxide and conversion efficiency to nitrogen dioxide.

FIG. 4 is a diagram showing NO concentrations in N measured according tothe embodiment as shown in FIG. 1.

In FIG. 1, a system, in which the present invention is applied to anapparatus for continuously analyzing NO, in the flue gas, isillustrated. A sample of flue gas 1 is passed through a cooling trap(dehumidifier) 2, a dust removal filter 3, and a throttling valve 4,whereby the flow rate of the sample is controlled. Then, the sample ispassed througha flow meter 6 and is led to a joint part 11 at a constantflow rate. On the other hand, an

oxygen cylinder 7 is controlled with respect to the-flow rate by athrottling valve 8, and oxygen is led from the oxygen cylinder 7 to anozone generator 10 at a constant flow rate through a flow meter 9. Atthat time, the flow rate of oxygen is set so that the moles of ozone inthe oxygen may be at least 1.5 times the maximum moles of NO in thesample gas. A gas mixture of oxygen and ozone leaving the ozonegenerator 10 is led to the joint part 11, and is brought into contactwith the sample gas there. N is reacted with ozone and oxidized to N 0Then, the sample gas mixture is passed through a heating furnance 12,where N 0 in the sample gas mixture is decomposed to N0 and excess ozoneis also decomposed to oxygen, and then led to a measurement cell 13 of adual wavelength, photoelectric type analyzer.

Now, experimental results when an'NO-mixed standard gas in N was used asa sample gas will be described below:

'As shown in FIG. 2, the absorption spectrum of N0 has a maximumabsorption at wavelength of about 400 my, and has no absorption almostat all at wavelength of 610 mu. Thus, the measurement wavelength of thedual wavelength photoelectric type analyzer was set to 405 my, and thereference wavelength to 610 mg.

The flow .rate of the sample gas was adjusted to constant 1 l/min., andthe flow rate of oxygen to constat 0.2 l/min., and the followingexperiment was carried out.

First of all, the optimum temperature of the heating furnance fordecomposing N 0 in the sample gas oxidized by ozone to N0 wasdetermined. Its experimental result is given in FIG. 3. 7

Heating furnace temperature of FIG. 3 is a temperature of heatingsection almost at the center of the heating furnace containing astainless steel pipe, 6 mm in outer diameter, which was properlyinsulated and wound with an electric heating wire and had a heatingsection, about 200 mm long. It was found that the optimum temperaturewas about 320 i C.

Then, indication characteristics were investigated by changing NOconcentrations in N sample gas. The heating furnace temperature was 320i 5C at that time. The experimental results are shown in FIG. 4.

As is clear from FIG. 4, the indication has a good linearity, and theconversion efficiency of NO to N0 is almost 100 percent. From theseresults, it is apparent that the method for conversion of the presentinvention is very effective.

In the present invention of converting NO, in the sample gas to N0optical analyzers for analyzing N0 such as two-light path,spectrophotometers of ultraviolet region and visible region having astandard cell and a measurement cell, or infra-red spectrophotometerscan be used in place of the dual wavelength, photoelectric type analyzerof ultra-violet region and visible region, as shown in FIG. 1.

Further, an intermittent colorimetric method using Saltzmann reagent asset forth in Japan Industrial Standard JIS K0104-l968 can be used in thepresent invention.

According to the present invention, the following effects can beobtained.

1. NO, in various sample gases can be converted to N0 continuously witha high efficiency by simple modification of the conventional analyzer,for example, by supplementing the conventional sampling system with anozone supply source and a heating furnace, and continuous analysis ispossible thereby. In other words, the maintenance is simple.

2. Optimum temperature range of the heating furnace is broad, and thusthe temperature control is simple.

3. Excess ozone is thermally decomposed to oxygen, and there is no fearof secondary pollution due to ozone.

4. It is possible to use oxygen from water electrolysis cell, etc. inplace of the oxygen from the oxygen cylinder.

5. It is not necessary at all to exchange an oxidizing agent.

6. Since dinitrogen pentaoxide is heated and decomposed to nitrogendioxide, and nitrogen dioxide is measured by an optical analyzer ofvisible range, the moisture in the nitrogen dioxide gas has no influenceat all, even the moisture remains therein.

The present invention is applicable to all the methods for analyzingnitrogen oxides through conversion of the nitrogen oxides to nitrogendioxide.

What is claimed is:

1. A method for analyzing the total nitrogen oxide content of a samplegas containing NO and N0 said method comprising:

a. reacting a sample gas containing NO and N0 with sufficient ozone gasto convert the NO and N0 to b. heating the gas so obtained andcontaining the resultant N 0 to decompose N 0 into N0 and O and c.continuously measuring the concentration of the N0 generated by thedecomposition of N 0 in the heated gas.

2. A method according to claim 1, wherein the amount of ozone gasreacted with said sample gas is at least 1.5 times as much as the amountof N0 and N0 in said sample gas on a molar basis.

3. A method according to claim 1, wherein the temperature to which thegas containing N 0 is heated is selected to maximize the conversionefficiency of the N 0 decomposition reaction.

4. A method according to claim 1, wherein the temperature to which thegas containing N 0 is heated is maintainedwithin a range such that theconversion efficiency of the N 0 decomposition reaction is keptsubstantially constant.

5. A method according to claim 4, wherein the temperature range for thedecomposition of N 0 into N0 and O is about 300C to 340C.

6. A method according to claim 1, wherein the concentration of NOresulting from the decomposition of N 0 by heating is optically measuredby absorbency of light beams having a specific wavelength.

7. A method according to claim 6, wherein at least one of said lightbeams has a wavelength from about 250 to 600mg. 7

8. A method according to claim 7, wherein at least one of said lightbeams has a wavelength of about 405mg 9. A. method according to claim 8,wherein another of said light beams has a wavelength outside of theabsorbence spectrum of N0 10. A method according to claim 9, whereinsaid another of said light beams has a wavelength of about 610mg.

11. A method according to claim 6, wherein the concentration of NOresulting from the decomposition of N by heating is measured by a dualwavelength spectrophotometer.

12. A method according to claim 11, wherein said dual wavelengthspectrophotometer covers the spectrum range of about 250 to 600mu.

13. A method according to claim 6, wherein the concentration of NOresulting from the decomposition of N 0 by heating is measured by atwo-light path spectrophotometer.

14. A method according to claim 13, wherein said two-light pathspectrophotometer covers the spectrum range of about 250 to 600m;.t.

15. A method according to claim 1, wherein the concentration of NOresulting from. the decomposition of N 0 by heating is measured byintermittent colorimetry.

16. A method according to claim 1, wherein the ozone gas reacted withsaid sample gas is in the form of a mixture of ozone gas and 0 gas.

17. An apparatus for analyzing the total nitrogen oxide concentration ofa sample gas containing NO and N0 said apparatus comprising:

a. sample inlet means for sampling a sample gas containing NO and N0 b.ozone supply means for supplying 0 gas,

0. mixing means for mixing sample gas containing NO and N0 from saidsample inlet means with 0 gas from said ozone supply means, therebyconverting NO and N0 into N 0 d. heating means connected to said mixingmeans for decomposing N 0 in the gas mixture into NO and O and e.measuring means connected to a downstream side of said heating means formeasuring the concentration of N0 generated by the decomposition of N 0in said heating means.

18. An apparatus according to claim 17, wherein said measuring means isan optical measuring means capable of measuring the absorbence of lightbeams having a specific wavelength.

19. The apparatus according to claim 18, wherein covering the spectrumrange of about 250 to 600m,u..

21. The apparatus according to claim 18, wherein said measuring means isa two-light path spectrophotometer.

22. The apparatus according to claim 21, wherein said two-light pathspectrophotometer is capable of covering the spectrum range of about250to 600mm.

. inlet means upstream of said measuring means for providing dust fromsaid sample gas.

25. An apparatus according to claim 18, further including fluidcontrolling means upstream of said mixing means for controlling themixing ratio of the sample gas and ozone gas mixed in said mixing means.

26. An apparatus according to claim 18, further including temperaturecontrol means for controlling the temperature of said heating means.

27. An apparatus according to claim 18, wherein said ozone supply meansincludes means for generating 0 said measuring means is a dualwavelength spectrophogas from 0 gas.

28. An apparatus according to claim 27, wherein said ozone supply meansfurther includes an 0 gas supply means.

29. An apparatus for analyzing the total nitrogen oxide concentration ofa sample gas containing NO and N0 said apparatus comprising:

a. sample inlet means for receiving a sample gas,

b. means for introducing 0 gas into said sample gas,

thereby converting NO and N0 into N 0 c. heating means fluidly connectedto said inlet means downstream of a point of introduction of the 0 gasfor decomposing N 0 in the resultant gas into N and O and d. measuringmeans connected to a downstream side of said heating means for measuringthe concentration of N0 generated by the decomposition of N 0 in saidheating means.

2. A method according to claim 1, wherein the amount of ozone gasreacted with said sample gas is at least 1.5 times as much as the amountof NO and NO2 in said sample gas on a molar basis.
 3. A method accordingto claim 1, wherein the temperature to which the gas containing N2O5 isheated is selected to maximize the conversion efficiency of the N2O5decomposition reaction.
 4. A method according to claim 1, wherein thetemperature to which the gas containing N2O5 is heated is maintainedwithin a range such that the conversion efficiency of the N2O5decomposition reaction is kept substantially constant.
 5. A methodaccording to claim 4, wherein the temperature range for thedecomposition of N2O5 into NO2 and O2 is about 300*C to 340*C.
 6. Amethod according to claim 1, wherein the concentration of NO2 resultingfrom the decomposition of N2O5 by heating is optically measured byabsorbency of light beams having a specific wavelength.
 7. A methodaccording to claim 6, wherein at least one of said light beams has awavelength from about 250 to 600m Mu .
 8. A method according to claim 7,wherein at least one of said light beams has a wavelength of about 405mMu .
 9. A method according to claim 8, wherein another of said lightbeams has a wavelength outside of the absorbence spectrum of NO2.
 10. Amethod according to claim 9, wherein said another of said light beamshas a wavelength of about 610m Mu .
 11. A method according to claim 6,wherein the concentration of NO2 resulting from the decomposition ofN2O5 by heating is measured by a dual wavelength spectrophotometer. 12.A method according to claim 11, wherein said dual wavelengthspectrophotometer covers the spectrum range of about 250 to 600m Mu .13. A method according to claim 6, wherein the concentration of NO2resulting from the decomposition of N2O5 by heating is measured by atwo-light path spectrophotometer.
 14. A method according to claim 13,wherein said two-light path spectrophotometer covers the spectrum rangeof about 250 to 600m Mu .
 15. A method according to claim 1, wherein theconcentration of NO2 resulting from the decomposition of N2O5 by heatingis measured by intermittent colorimetry.
 16. A method according to claim1, wherein the ozone gas reacted with said sample gas is in the form ofa mixture of ozone gas and O2 gas.
 17. An apparatus for analyzing thetotal nitrogen oxide concentration of a sample gas containing NO andNO2, said apparatus comprising: a. sample inlet means for sampling asample gas containing NO and NO2, b. ozone supply means for supplying O3gas, c. mixing means for mixing sample gas containing NO and NO2 fromsaid sample inlet means with O3 gas from said ozone supply means,thereby converting NO and NO2 into N2O5, d. heating means connected tosaid mixing means for decomposing N2O5 in the gas mixture into NO2andO2, and e. measuring means connected to a downstream side of saidheating means for measuring the concentration of NO2 generated by thedecomposition of N2O5 in said heating means.
 18. An apparatus accordingto claim 17, wherein said measuring means is an optical measuring meanscapable of measuring the absorbence of light beams having a specificwavelength.
 19. The apparatus according to claim 18, wherein saidmeasuring means is a dual wavelength spectrophotometer.
 20. Theapparatus according to claim 19, wherein said dual wavelengthspectrophotometer is capable of covering the spectrum range of about 250to 600m Mu .
 21. The apparatus according to claim 18, wherein saidmeasuring means is a two-light path spectrophotometer.
 22. The apparatusaccording to claim 21, wherein said two-light path spectrophotometer iscapable of covering the spectrum range of about 250 to 600m Mu .
 23. Theapparatus according to claim 18, further including a demoistening meansfluidly connected to said sample inlet means upstream of said measuringmeans for removing moisture from the same gas containing NO and NO2. 24.The apparatus according to claim 18, further including filtering meansfluidly connected to said sample inlet means upstream of said measuringmeans for providing dust from said sample gas.
 25. An apparatusaccording to claim 18, further including fluid controlling meansupstream of said mixing means for controlling the mixing ratio of thesample gas and ozone gas mixed in said mixing means.
 26. An apparatusaccording to claim 18, further including temperature control means forcontrolling the temperature of said heating means.
 27. An apparatusaccording to claim 18, wherein said ozone supply means includes meansfor generating O3 gas from O2 gas.
 28. An apparatus according to claim27, wherein said ozone supply means further includes an O2 gas supplymeans.
 29. An apparatus for analyzing the total nitrogen oxideconcentration of a sample gas containing NO and NO2, said apparatuscomprising: a. sample inlet means for receiving a sample gas, b. meansfor introducing O3 gas into said sample gas, thereby converting NO andNO2 into N2O5, c. heating means fluidly connected to said inlet meansdownstream of a point of introduction of the O3 gas for decomposing N2O5in the resultant gas into N2 and O2, and d. measuring means connected toa downstream side of said heating means for measuring the concentrationof NO2 generated by the decomposition of N2O5 in said heating means.